Chemical mechanical polishing apparatus with non-conductive elements

ABSTRACT

Conductive elements of a chemical mechanical polishing system may generate undesired eddy currents under the influence of a time-dependent magnetic field used in an eddy current monitoring system. To improve the accuracy of an eddy current monitoring system, elements that may contribute an undesired signal to the sensed eddy current signal may be fabricated from a non-conductive material such as plastic or ceramic. In some implementations, elements may be fabricated from non-magnetic materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application and claims the benefit ofpriority under 35 U.S.C. Section 120 of U.S. application Ser. No.10/643,773, filed on Aug. 18, 2003, which claims the benefit of priorityof U.S. Provisional Application Ser. No. 60/452,406, filed Mar. 4, 2003.The disclosure of each prior application is considered part of and isincorporated by reference in the disclosure of this application.

BACKGROUND

This invention relates to semiconductor manufacturing, and moreparticularly to endpoint detection.

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive or insulative layerson a silicon wafer. One fabrication step involves depositing a fillerlayer over a non-planar surface, and planarizing the filler layer untilthe non-planar surface is exposed. For example, a conductive fillerlayer can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. The filler layer is thenpolished until the raised pattern of the insulative layer is exposed.After planarization, the portions of the conductive layer remainingbetween the raised pattern of the insulative layer form vias, plugs andlines that provide conductive paths between thin film circuits on thesubstrate. In addition, planarization is needed to planarize thesubstrate surface for photolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is placed against a rotating polishing disk pad or beltpad. The polishing pad can be either a “standard” pad or afixed-abrasive pad. A standard pad has a durable roughened surface,whereas a fixed-abrasive pad has abrasive particles held in acontainment media. The carrier head provides a controllable load on thesubstrate to push it against the polishing pad. A polishing slurry,including at least one chemically-reactive agent, and abrasive particlesif a standard pad is used, is supplied to the surface of the polishingpad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Overpolishing (removing too much) of a conductive layer orfilm may lead to increased circuit resistance. On the other hand,underpolishing (removing too little) of a conductive layer may lead toelectrical shorting. Variations in the initial thickness of thesubstrate layer, the slurry composition, the polishing pad condition,the relative speed between the polishing pad and the substrate, and theload on the substrate can cause variations in the material removal rate.These variations cause variations in the time needed to reach thepolishing endpoint. Therefore, the polishing endpoint cannot bedetermined merely as a function of polishing time.

One way to determine the polishing endpoint is to remove the substratefrom the polishing surface and examine it. For example, the substratecan be transferred to a metrology station where the thickness of asubstrate layer is measured, e.g., with a profilometer or a resistivitymeasurement. If the desired specifications are not met, the substrate isreloaded into the CMP apparatus for further processing. This is atime-consuming procedure that reduces the throughput of the CMPapparatus. Alternatively, the examination might reveal that an excessiveamount of material has been removed, rendering the substrate unusable.

More recently, in-situ monitoring of the substrate has been performed,e.g., with optical or capacitance sensors, in order to detect thepolishing endpoint. Other proposed endpoint detection techniques haveinvolved measurements of friction, motor current, slurry chemistry,acoustics and conductivity. One detection technique that has beenconsidered is to induce an eddy current in the metal layer and measurethe change in the eddy current as the metal layer is removed.

SUMMARY

In general, in one aspect a carrier head may include a non-conductivesubstrate backing assembly, which may include a flexible membrane andone or more clamp rings. The carrier head may include a base assembly,where some components of the base assembly may be non-conductive. Thecarrier head may include a housing, which may include non-conductiveelements. Portions of the carrier head within a sensing distance of thesubstrate mounting surface may be non-conductive. The sensing distancemay be between about one tenth of an inch and about two inches,depending on a number of factors.

The non-conductive elements of the carrier head may also benon-magnetic. That is, they may have a relatively small magneticpermeability and a relatively large resistivity. In someimplementations, some elements may be conductive but non-magnetic. Forexample, non-magnetic fasteners such as aluminum fasteners may be usedrather than magnetic fasteners such as steel fasteners.

In general, in another aspect, a polishing system may include apolishing pad having a polishing surface, a carrier to hold a substrateagainst the polishing surface of the polishing pad, and an eddy currentmonitoring system including an induction coil positioned on a side ofthe polishing surface opposite the substrate. The induction coil may beto generate a magnetic field through the pad into a sensing region ofthe system. Components of the polishing system with at least a portionpositioned within a sensing distance of the polishing pad in the sensingregion may be non-conductive. The sensing distance may be a distancebeyond which the eddy current signal from one or more conductivecomponents of the system is not discernible over a noise signal. Thesensing distance may be a distance beyond which the eddy current signalfrom the one or more conductive components of the system in the sensingregion is about equal to or less than an error amount corresponding toan acceptable amount of sign inaccuracy.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic exploded perspective view of a chemical mechanicalpolishing apparatus.

FIG. 2A is a schematic side view, partially cross-sectional, of achemical mechanical polishing apparatus including an eddy currentmonitoring system.

FIG. 2B is a schematic top view of a chemical mechanical polishingapparatus including an eddy current monitoring system, showing a path ofa sensor scan across a wafer.

FIG. 3 is a schematic circuit diagram of the eddy current monitoringsystem.

FIGS. 4A-4C are schematic cross-sectional views of a polishing pad.

FIG. 5 is a schematic cross-sectional view illustrating a magnetic fieldgenerated by the monitoring system.

FIG. 6 is a schematic perspective view of a core from an eddy currentsensor.

FIGS. 7A-7D schematically illustrating a method of detecting a polishingendpoint using an eddy current sensor.

FIG. 8 is a graph illustrating a trace from the eddy current monitoringsystem.

FIG. 9 is a schematic diagrams an eddy current monitoring system thatsenses a phase shift.

FIGS. 10A and 10B are schematic circuit diagrams of two implementationsof an eddy current monitoring system of FIG. 9.

FIG. 11 is a graph illustrating a trace from the eddy current monitoringsystem that measures phase shift.

FIG. 12 is a cross-sectional view of a simplified representation of anelectromagnetic field distribution relative to an eddy current sensorsystem and a substrate in a chemical mechanical polishing apparatus.

FIG. 13 is a cross-sectional view of a carrier head for a chemicalmechanical polishing apparatus.

FIG. 14 is a cross-sectional view of another carrier head for a chemicalmechanical polishing apparatus.

FIG. 15 is a cross-sectional view of another carrier head for a chemicalmechanical polishing apparatus.

FIG. 16 is a plot of eddy current signal versus time for twoimplementations of a chemical mechanical polishing system.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

The current disclosure provides methods and apparatus for improving aneddy current sensing system by providing non-conductive and/ornon-magnetic elements in regions where conductive or magnetic elementsmay affect the eddy current signal.

In a chemical mechanical polishing apparatus with an eddy currentmonitoring system, changes in a conductive layer on a wafer may bemonitored by detecting an amplitude and/or a phase of a received signal.In some implementations, an amplitude signal may be more sensitive tochanges in polishing pad thickness (e.g., due to pad wear or swelling)than a phase signal. Because of this effect, detecting a phase signalmay provide a more accurate measure of changes in the conductive layer.

However, the phase signal may be more susceptible to the effect of eddycurrents generated in regions outside the conductive region of interest.For example, the phase signal may be non-monotonic (i.e., two differentconductive layer thicknesses may correspond to the same phase value) dueto eddy currents generated in the chemical mechanical polishing systemrather than in the conductive region on the wafer.

Therefore, in order to provide an eddy current sensing signal that moreaccurately reflects the thickness of one or more conductive regions on awafer being polished, the current application describes a CMP apparatuswhere those portions of a CMP carrier head that may prevent a suitablyaccurate measurement of a conductive layer on a wafer are fabricatedusing non-conductive materials and/or non-magnetic materials (materialswith low magnetic permeability). For example, parts of a CMP carrierhead that are proximate to a substrate during polishing may befabricated from non-conductive and/or non-magnetic materials rather thanconductive, magnetic materials such as steel.

Reducing extraneous contributions from the sensed eddy current signal isparticularly important in emerging systems that use real-time profilecontrol. In real-time profile control, the sensed eddy current signal isused to update polishing parameters in real time. Noise in the eddycurrent signal may prevent the real-time profile control system fromaccurately controlling polishing parameters.

Referring to FIGS. 1 and 2A, one or more substrates 10 can be polishedby a CMP apparatus 20. A description of a similar polishing apparatus 20can be found in U.S. Pat. No. 5,738,574, the entire disclosure of whichis incorporated herein by reference. Polishing apparatus 20 includes aseries of polishing stations 22 and a transfer station 23. Transferstation 23 transfers the substrates between the carrier heads and aloading apparatus.

Each polishing station includes a rotatable platen 24 on which is placeda polishing pad 30. The first and second stations can include atwo-layer polishing pad with a hard durable outer surface or afixed-abrasive pad with embedded abrasive particles. The final polishingstation can include a relatively soft pad. Each polishing station canalso include a pad conditioner apparatus 28 to maintain the condition ofthe polishing pad so that it will effectively polish substrates.

A rotatable multi-head carousel 60 supports four carrier heads 70. Thecarousel is rotated by a central post 62 about a carousel axis 64 by acarousel motor assembly (not shown) to orbit the carrier head systemsand the substrates attached thereto between polishing stations 22 andtransfer station 23. Three of the carrier head systems receive and holdsubstrates, and polish them by pressing them against the polishing pads.Meanwhile, one of the carrier head systems receives a substrate from anddelivers a substrate to transfer station 23.

Each carrier head 70 is connected by a carrier drive shaft 74 to acarrier head rotation motor 76 (shown by the removal of one quarter ofcover 68) so that each carrier head can independently rotate about itown axis. In addition, each carrier head 70 independently laterallyoscillates in a radial slot 72 formed in carousel support plate 66. Adescription of a suitable carrier head 70 can be found in U.S. Pat. No.6,183,354, filed May 21, 1997, issued Feb. 6, 2001, the entiredisclosure of which is incorporated herein by reference. A descriptionof other carrier heads may be found below. In operation, the platen isrotated about its central axis 25, and the carrier head is rotated aboutits central axis 71 and translated laterally across the surface of thepolishing pad.

A slurry 38 containing one or more chemically reactive agents such ascatalyzers and oxidizers can be supplied to the surface of polishing pad30 by a slurry supply port or combined slurry/rinse arm 39. If polishingpad 30 is a standard pad, slurry 38 can also include abrasive particles.

When a CMP apparatus is removing conductive material from the surface ofa substrate, an eddy current monitoring system may be used to monitorchanges in one or more conductive regions. Referring to FIGS. 2A and 3,an in-situ eddy current monitoring system may be provided in a chemicalmechanical polishing system. A recess 26 is formed in platen 24, and athin section 36 can be formed in polishing pad 30 overlying recess 26.Aperture 26 and thin pad section 36, if needed, are positioned such thatthey pass beneath substrate 10 during a portion of the platen'srotation, regardless of the translational position of the carrier head.Assuming that polishing pad 32 is a two-layer pad, thin pad section 36can be constructed as shown in FIG. 4A by removing a portion 33 ofbacking layer 32.

Alternatively, as shown in FIG. 4B, thin pad section 36′ can be formedby removing a portion 33′ of both backing layer 32′ and a portion ofcover layer 34′. Thus, this implementation has a recess in the bottomsurface of cover layer 34 in the thin pad section 36. If the polishingpad is a single-layer pad, thin pad section 36 can be formed by removinga portion of the pad material to create a recess in the bottom surfaceof the pad.

Alternatively, as shown in FIG. 4C, thin pad section 36″ can be formedby inserting a plug 37 of a different material into polishing pad 30.For example, the plug can be a relatively pure polymer or polyurethane,e.g., formed without fillers. In general, the material of pad section 36should be non-magnetic and non-conductive. If the polishing pad isitself sufficiently thin or has a magnetic permeability (andconductivity) that does not interfere with the eddy currentmeasurements, then the pad does not need any modifications or recesses.

Returning to FIGS. 2A and 3, an in-situ eddy current monitoring system40, which can function as an endpoint detector, includes a drive system48 to induce eddy currents in a metal layer on the substrate and asensing system 58 to detect eddy currents induced in the metal layer bythe drive system. The monitoring system 40 includes a core 42 positionedin recess 26 to rotate with the platen, a drive coil 44 wound around onepart of core 42, and a sense coil 46 wound around a second part of core42. For drive system 48, monitoring system 40 includes an oscillator 50connected to drive coil 44. For sense system 58, monitoring system 40includes a capacitor 52 connected in parallel with sense coil 46, an RFamplifier 54 connected to sense coil 46, and a diode 56. The oscillator50, capacitor 52, RF amplifier 54, and diode 56 can be located apartfrom platen 24, and can be coupled to the components in the platenthrough a rotary electrical union 29.

Referring to FIG. 5, in operation the oscillator 50 drives drive coil 44to generate an oscillating magnetic field 48 that extends through thebody of core 42 and into the gap between the two poles 42 a and 42 b ofthe core. At least a portion of magnetic field 48 extends through thinportion 36 of a polishing pad and into substrate 10. If a metal layer 12is present on substrate 10, oscillating magnetic field 48 generates eddycurrents in the metal layer 12. The eddy currents cause the metal layer12 to act as an impedance source in parallel with sense coil 46 andcapacitor 52. As the thickness of the metal layer changes, the impedancechanges, resulting in a change in the Q-factor of sensing mechanism. Bydetecting the change in the Q-factor of the sensing mechanism, the eddycurrent sensor can sense the change in the strength of the eddycurrents, and thus the change in thickness of metal layer 12.

In operation, CMP apparatus 20 uses monitoring system 40 to determinewhen the bulk of the filler layer has been removed and the underlyingstop layer has been exposed. Monitoring system 40 can be used todetermine the amount of material removed from the surface of thesubstrate. A general purpose programmable digital computer 90 can beconnected to amplifier 56 to receive the intensity signal from the eddycurrent sensing system. Computer 90 can be programmed to sampleamplitude measurements from the monitoring system when the substrategenerally overlies the core, to store the amplitude measurements, and toapply the endpoint detection logic to the measured signals to detect thepolishing endpoint. Possible endpoint criteria for the detector logicinclude local minima or maxima, changes in slope, threshold values inamplitude or slope, or combinations thereof.

Referring to FIG. 2B, the core 42, drive coil 44 and sense coil 46 ofthe eddy current sensor located below thin section 36 of polishing pad32 sweep beneath the substrate with each rotation of the platen.Therefore, the computer 90 can also be programmed to divide theamplitude measurements from each sweep of the core beneath the substrateinto a plurality of sampling zones 96, to calculate the radial positionof each sampling zone, to sort the amplitude measurements into radialranges, to determine minimum, maximum and average amplitude measurementsfor each sampling zone, and to use multiple radial ranges to determinethe polishing endpoint, as discussed in U.S. patent application Ser. No.09/460,529, filed Dec. 13, 1999, the entirety of which is incorporatedherein by reference.

Since the eddy current sensor sweeps beneath the substrate with eachrotation of the platen, information on the metal layer thickness isbeing accumulated in-situ and on a continuous real-time basis. In fact,the amplitude or phase (or both) measurements from the eddy currentsensor can be displayed on an output device 92 during polishing topermit the operator of the device to visually monitor the progress ofthe polishing operation.

Moreover, after sorting the amplitude measurements into radial ranges,information on the metal film thickness can be fed in real-time into aclosed-loop controller to periodically or continuously modify thepolishing pressure profile applied by a carrier head, as discussed inU.S. patent application Ser. No. 60/143,219, filed Jul. 7, 1999, theentirety of which is incorporated herein by reference. For example, thecomputer could determine that the endpoint criteria have been satisfiedfor the outer radial ranges but not for the inner radial ranges. Thiswould indicate that the underlying layer has been exposed in an annularouter area but not in an inner area of the substrate. In this case, thecomputer could reduce the diameter of the area in which pressure isapplied so that pressure is applied only to the inner area of thesubstrate, thereby reducing dishing and erosion on the outer area of thesubstrate. Alternatively, the computer can halt polishing of thesubstrate on the first indication that the underlying layer has beenexposed anywhere on the substrate, i.e., at first clearing of the metallayer.

Initially, referring to FIGS. 2A, 3 and 7A, oscillator 50 is tuned tothe resonant frequency of the LC circuit, without any substrate present.This resonant frequency results in the maximum amplitude of the outputsignal from RF amplifier 54.

As shown in FIGS. 7B and 8, for a polishing operation, a substrate 10 isplaced in contact with polishing pad 30. Substrate 10 can include asilicon wafer 12 and a conductive layer 16, e.g., a metal such ascopper, disposed over one or more patterned underlying layers 14, whichcan be semiconductor, conductor or insulator layers. The patternedunderlying layers can include metal features, e.g., vias, pads andinterconnects. Since, prior to polishing, the bulk of conductive layer16 is initially relatively thick and continuous, it has a lowresistivity, and relatively strong eddy currents can be generated in theconductive layer. As previously mentioned, the eddy currents cause themetal layer to function as an impedance source in parallel with sensecoil 46 and capacitor 52.

Consequently, the presence of conductive film 16 reduces the Q-factor ofthe sensor circuit, thereby significantly reducing the amplitude of thesignal from RF amplifier 56.

Referring to FIGS. 7C and 8, as substrate 10 is polished, the bulkportion of conductive layer 16 is thinned. As the conductive layer 16thins, its sheet resistivity increases, and the eddy currents in themetal layer become dampened. Consequently, the coupling between metallayer 16 and sensor circuitry 58 is reduced (i.e., increasing theresistivity of the virtual impedance source). As the coupling declines,the Q-factor of the sensor circuit 58 increases toward its originalvalue.

Referring to FIGS. 7D and 8, eventually the bulk portion of conductivelayer 16 is removed, leaving conductive interconnects 16′ in thetrenches between the patterned insulative layer 14. At this point, thecoupling between the conductive portions in the substrate, which aregenerally small and generally non-continuous, and sensor circuit 58reaches a minimum. Consequently, the Q-factor of the sensor circuitreaches a maximum value (although not as large as the Q-factor when thesubstrate is entirely absent). This causes the amplitude of the outputsignal from the sensor circuit to plateau. Thus, by sensing when theamplitude of the output signal is no longer increasing and has leveledoff (e.g., reached a local plateau), computer 90 can sense a polishingendpoint. Alternatively, by polishing one or more test substrates, theoperator of the polishing machine can determine the amplitude of theoutput signal as a function of the thickness of the metal layer. Thus,the endpoint detector can halt polishing when a particular thickness ofthe metal layer remains on the substrate. Specifically, computer 90 cantrigger the endpoint when the output signal from the amplifier exceeds avoltage threshold corresponding to the desired thickness.

The eddy current monitoring system can also be used to trigger a changein polishing parameters. For example, when the monitoring system detectsa polishing criterion, the CMP apparatus can change the slurrycomposition (e.g., from a high-selectivity slurry to a low selectivityslurry). As another example, as discussed above, the CMP apparatus canchange the pressure profile applied by the carrier head.

In addition to sensing changes in amplitude, the eddy current monitoringsystem can calculate a phase shift in the sensed signal. As the metallayer is polished, the phase of the sensed signal changes relative tothe drive signal from the oscillator 50. This phase difference can becorrelated to the thickness of the polished layer. One implementation ofa phase measuring device, shown in FIG. 10A, combines the drive andsense signals to generate a phase shift signal with a pulse width orduty cycle which is proportional to the phase difference. In thisimplementation, two XOR gates 100 and 102 are used to convert sinusoidalsignals from the sense coil 46 and oscillator 50, respectively, intosquare-wave signals. The two square-wave signals are fed into the inputsof a third XOR gate 104. The output of the third XOR gate 104 is a phaseshift signal with a pulse width or duty cycle proportional to the phasedifference between the two square wave signals. The phase shift signalis filtered by an RC filter 106 to generate a DC-like signal with avoltage proportional to the phase difference. Alternatively, the signalscan be fed into a programmable digital logic, e.g., a ComplexProgrammable Logic Device (CPLD) or Field Programmable Gate Array (FGPA)that performs the phase shift measurements.

The phase shift measurement can be used to detect the polishing endpointin the same fashion as the amplitude measurements discussed above.Alternatively, both amplitude and phase shift measurements could be usedin the endpoint detection algorithm. An implementation for both theamplitude and phase shift portions of the eddy current monitoring systemis shown in FIG. 10A. An implementation of the amplitude sensing portionof the eddy current monitoring system is shown in FIG. 10B. An exampleof a trace generated by an eddy current monitoring system that measuresthe phase difference between the drive and sense signals is shown inFIG. 11. Since the phase measurements are highly sensitive to thestability of the driving frequency, phase locked loop electronics may beadded.

A possible advantage of the phase difference measurement is that thedependence of the phase difference on the metal layer thickness may bemore linear than that of the amplitude. In addition, the absolutethickness of the metal layer may be determined over a wide range ofpossible thicknesses. A phase difference measurement may additionally beless sensitive to changes in pad thickness than an amplitudemeasurement.

The eddy current monitoring system can be used in a variety of polishingsystems. Either the polishing pad, or the carrier head, or both can moveto provide relative motion between the polishing surface and thesubstrate. The polishing pad can be a circular (or some other shape) padsecured to the platen, a tape extending between supply and take-uprollers, or a continuous belt. The polishing pad can be affixed on aplaten, incrementally advanced over a platen between polishingoperations, or driven continuously over the platen during polishing. Thepad can be secured to the platen during polishing, or there could be afluid bearing between the platen and polishing pad during polishing. Thepolishing pad can be a standard (e.g., polyurethane with or withoutfillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather thantuning when the substrate is absent, the drive frequency of theoscillator can be tuned to a resonant frequency with a polished orunpolished substrate present (with or without the carrier head), or tosome other reference.

Various aspects of the invention, such as placement of the coil on aside of the polishing surface opposite the substrate or the measurementof a phase difference, still apply if the eddy current sensor uses asingle coil. In a single coil system, both the oscillator and the sensecapacitor (and other sensor circuitry) are connected to the same coil.

In an implementation of a semiconductor processing apparatus, an in-situeddy current monitoring system such as system 40 of FIGS. 2A and 3 maybe used to monitor the thickness of a conductive layer and/or to detectan endpoint or other point in a semiconductor process. The monitoringsystem may include a sensing system such as sensing system 58 to detecteddy currents induced in a conductive layer, using a drive system suchas drive system 48. In some implementations, a core for a sensing systemmay be positioned in a recess in a platen.

In order to obtain information about properties of a conductive layer ona substrate, a time-dependent magnetic field may be produced using thedrive system of the eddy current monitoring system. As explained above,a conductive layer acts as an impedance source and reduces the receivedsignal. In order to provide an accurate measure of the thickness of aconductive layer (or accurate endpoint determination), the magneticfield needs to have sufficient magnitude at the conductive layer so thatit can have a measurable effect on the received signal. For conductivelayers with higher resistivities (e.g., tungsten layers rather thancopper layers), the magnetic field may need to have a greater amplitude,since the magnitude of produced eddy currents is smaller.

However, in order to provide a sufficient magnetic field in theconductive region of interest, the magnetic field may have anon-negligible amplitude in conductive and/or magnetic regions of thesemiconductor processing apparatus other than the conductive regions ofinterest. In such cases, inaccuracies may be introduced into thereceived signal.

Referring to FIG. 12, an eddy current monitoring sensor assembly 1200produces a varying magnetic field 1205 in order generate eddy currentsin one or more conductive regions 1245 on a substrate 1240, in order tomonitor the thickness of the conductive regions 1245. Assembly 1200includes a drive coil 1210 and a sense coil 1230 around a core 1220 forproducing field 1205. The profile of magnetic field 1205 is generallydetermined by the geometry of eddy current sensor assembly 1200; forexample, core shape, orientation of windings, shielding, and presence ofconductive or magnetic material proximate to the sensor. The maximumvalue of the time-dependent current in coil 1210 may be selected basedon factors such as the resistivity of the material of conductive regions1245. For example, in order to monitor the thickness of a relativelyresistive conductive region 1245 (e.g., tungsten rather than copper),the eddy current monitoring sensor assembly 1200 may provide morecurrent to coil 1210 to produce a time-varying magnetic field of greatermaximum magnitude. Depending on the details of assembly 1200, themagnetic field may have a non-negligible magnitude beyond conductiveregion 1245.

During polishing, substrate 1240 is held against a polishing pad 1250having a thin portion 1255 by a flexible membrane 1260. A plate 1270proximate to the flexible membrane may be included in a carrier headassembly (e.g., plate 1410 of carrier head 1400 FIG. 14 or carrier body1526 of carrier head 1500 of FIG. 15). FIG. 12 illustrates animplementation where the magnetic field generated by coil 1210 has anon-negligible magnitude at plate 1270. If plate 1270 is fabricatedusing a conductive and/or magnetic material such as a metal, plate 1270will generate eddy currents, which may affect the signal received atsense coil 1230, reducing the accuracy of the thickness or end pointmeasurement being made. Therefore, fabricating plate 1270 from anon-conductive, non-magnetic material such as plastic or ceramic mayimprove the accuracy of the eddy current monitoring system.

For a particular chemical mechanical polishing system using an eddycurrent monitoring sensor assembly 1200, a sensing distance D may bedefined between, for example, a surface of core 1220 and a plane 1280,where portions of the chemical mechanical polishing system at a distanceof D or less from the surface of core 1220 are non-conductive, in orderto improve the sensing ability of the eddy current monitoring system.

The sensing distance D may correspond to a distance at which the eddycurrent signal generated in conductive parts of the chemical mechanicalpolishing system rather than in conductive regions on the substrate isnot discernible over other noise in the signal. Alternately, D may bechosen as a distance at which the eddy current signal generated inconductive parts of the system is small enough that the accuracy of thethickness or endpoint measurement being made falls within acceptablelimits. For example, an error amount may be defined for a measurement ofan eddy current signal. D may be chosen as a distance beyond which theeddy current signal generated in conductive parts of the system is lessthan or equal to the error amount. D may be chosen in some other way;for example, as a distance at which the magnetic field falls to acertain percentage of the maximum value.

D may depend on a number of factors, including the types of conductivematerials to be polished, the geometry of the eddy current monitoringsystem, and the acceptable contribution to the sensed signals fromsources other than the conductive regions on the layer. For magneticfields more localized in the substrate region, a smaller sensingdistance may suffice; for example, the signal accuracy may be acceptablewhen parts of the carrier head within about a tenth of an inch of thesubstrate are non-conductive. For magnetic fields having an appreciablemagnitude beyond the substrate, a larger sensing distance such as asensing distance between about one inches and about two inches or evengreater may be necessary to achieve a desired signal accuracy.

Determining which elements of a chemical mechanical polishing systemshould be non-conductive/non-magnetic depends on the system being used.Different implementations of carrier heads may have different elementsthat may potentially reduce the accuracy of the eddy current sensingsystem. Referring to FIG. 13, a carrier head 1300 that may be used in aCMP apparatus such as apparatus 20 of FIG. 1 includes a housing 1302, abase assembly 1304, a gimbal mechanism 1306 (which may be consideredpart of the base assembly), a loading chamber 1308, a retaining ring1310, and a substrate backing assembly 1312 which includes fivepressurizable chambers. A description of a similar carrier head may befound in U.S. patent application Ser. No. 09/712,389, “Multi-ChamberCarrier Head with a Flexible Membrane,” filed Nov. 13, 2000, which ishereby incorporated by reference.

The housing 1302 can be generally circular in shape and can be connectedto the drive shaft 74 of FIG. 1 to rotate therewith during polishing. Avertical bore 1320 may be formed through the housing 1302, and fiveadditional passages 1322 (only two passages are illustrated) may extendthrough the housing 1302 for pneumatic control of the carrier head.O-rings 1324 may be used to form fluid-tight seals between the passagesthrough the housing and passages through the drive shaft.

The base assembly 1304 is a vertically movable assembly located beneaththe housing 1302. The base assembly 1304 includes a main base portionsuch as a generally rigid annular body 1330, an outer clamp ring 1334,and the gimbal mechanism 1306. The gimbal mechanism 1306 includes agimbal rod 1336 which slides vertically the along bore 1320 to providevertical motion of the base assembly 1304, and a flexure ring 1338 whichbends to permit the base assembly to pivot with respect to the housing1302 so that the retaining ring 1310 may remain substantially parallelwith the surface of the polishing pad.

The loading chamber 1308 is located between the housing 1302 and thebase assembly 1304 to apply a load, i.e., a downward pressure or weight,to the base assembly 1304. The vertical position of the base assembly1304 relative to the polishing pad 32 of FIG. 1 is also controlled bythe loading chamber 1308. An inner edge of a generally ring-shapedrolling diaphragm 1326 may be clamped to the housing 1302 by an innerclamp ring 1328. An outer edge of the rolling diaphragm 1326 may beclamped to the base assembly 1304 by the outer clamp ring 1334.

The retaining ring 1310 may be a generally annular ring secured at theouter edge of the base assembly 1304. When fluid is pumped into theloading chamber 1308 and the base assembly 1304 is pushed downwardly,the retaining ring 1310 is also pushed downwardly to apply a load to thepolishing pad 32 of FIG. 1. A bottom surface 1316 of the retaining ring1310 may be substantially flat, or it may have a plurality of channelsto facilitate transport of slurry from outside the retaining ring to thesubstrate. An inner surface 1318 of the retaining ring 1310 engages thesubstrate to prevent it from escaping from beneath the carrier head.

The substrate backing assembly 1312 includes a flexible membrane 1340with a generally flat main portion 1342 and five concentric annularflaps 1350, 1352, 1354, 1356, and 1358 extending from the main portion1342. The edge of the outermost flap 1358 is clamped between the baseassembly 1304 and a first clamp ring 1346. Two other flaps 1350, 1352are clamped to the base assembly 1304 by a second clamp ring 1347, andthe remaining two flaps 1354 and 1356 are clamped to the base assembly1304 by a third clamp ring 1348. A lower surface 1344 of the mainportion 1342 provides a mounting surface for the substrate 10.

The volume between the base assembly 1304 and the internal membrane 1350that is sealed by the first flap 1350 provides a first circularpressurizable chamber 1360. The volume between the base assembly 1304and the internal membrane 1350 that is sealed between the first flap1350 and the second flap 1352 provides a second pressurizable annularchamber 1362 surrounding the first chamber 1360. Similarly, the volumebetween the second flap 1352 and the third flap 1354 provides a thirdpressurizable chamber 1364, the volume between the third flap 1354 andthe fourth flap 1356 provides a fourth pressurizable chamber 1366, andthe volume between the fourth flap 1356 and the fifth flap 1358 providesa fifth pressurizable chamber 1368. As illustrated, the outermostchamber 1368 is the narrowest chamber. In fact, the chambers 1352, 1354,1356 and 1358 can be configured to be successively narrower.

Each chamber can be fluidly coupled by passages through the baseassembly 1304 and housing 1302 to an associated pressure source, such asa pump or pressure or vacuum line. One or more passages from the baseassembly 1304 can be linked to passages in the housing by flexibletubing that extends inside the loading chamber 1308 or outside thecarrier head. Thus, pressurization of each chamber, and the forceapplied by the associated segment of the main portion 1342 of theflexible membrane 1340 on the substrate, can be independentlycontrolled. This permits different pressures to be applied to differentradial regions of the substrate during polishing, thereby compensatingfor non-uniform polishing rates caused by other factors or fornon-uniform thickness of the incoming substrate.

Depending on the design of the eddy current sensing system, one or moreparts of a carrier head such as carrier head 1300 of FIG. 13 maycontribute to the received signal, and therefore affect the accuracy ofthe reading. In order to prevent such effects, elements of the chemicalmechanical polishing apparatus which may produce eddy currents inresponse to the drive signal of the eddy current sensing system may benon-conductive and non-magnetic (i.e., have a high resistivity and a lowmagnetic permeability).

Conductive/magnetic elements of carrier head 1300 in higher fieldregions may provide a greater contribution to the received signal. Thoseelements that are less resistive (e.g., fabricated from a material witha lower resistivity and/or having a shorter length/smaller crosssectional area for current flow) may provide a greater contribution tothe received signal, as may those elements having a greater magneticpermeability. Therefore, elements of carrier head 1300 that are closerto substrate 10 and/or are larger may be fabricated from anon-conductive material to improve the ability of the eddy currentsensing system to reflect changes in one or more conductive regions on asubstrate.

In some implementations, the eddy current signal may be sufficientlyimproved by fabricating elements of carrier 1300 using semiconductivematerials such as silicon or semi-conductive ceramics. Additionally, insome implementations replacing conductive, magnetic parts withconductive non-magnetic parts may provide a significant signalimprovement. For example, some stainless steel parts made from an alloywith a non-negligible magnetic permeability may be replaced by aluminumparts. Although the resistivity of aluminum is lower than stainlesssteel, aluminum is non-magnetic and generally has a less detrimentaleffect on the measured signal.

In FIG. 13, some or all of the elements comprising base assembly 1304may be non-conductive. Some or all of gimbal mechanism 1306 may benon-conductive. Additionally, some or all of housing 1302 may benon-conductive. Some or all fasteners (e.g., bolts) for assemblingcarrier 1300 (not shown in FIG. 13), such as the fasteners that securethe retaining ring 1310 to the base 1304, as well as supports formembrane 1340 may be non-conductive.

Providing non-conductive/non-magnetic fasteners may eliminate irregularnoise termed “screw bump” noise. A particular fastener may contribute tothe sensed signal in scans where the sensor scans under the screw, butnot in other scans. Whether or not a particular fastener contributes tothe signal depends on the geometry of the system, the rotational speedof the platen/head, and the head sweep. A screw bump is particularlyproblematic, because it may be confused with a signal caused by alocally thicker copper layer.

For a particular implementation of a chemical mechanical polishingsystem with an eddy current monitoring system, a minimum distance suchas the sensing distance D discussed above may be determined, whereunshielded conductive elements within the minimum distance of the eddycurrent monitoring system will detrimentally contribute to the eddycurrent signal. Although D was defined in terms of a distance from asurface of the sensing core, the minimum distance could alternately bestated in terms of the distance from the bottom of the flexible membrane(which is about equal to the distance from a conductive region on thewafer during polishing of the conductive region).

As mentioned above, some carrier head assemblies include a plate or ringbehind a flexible membrane that holds a substrate to the polishing pad,where the plate may be perforated. The plate is generally close to thesubstrate during processing (e.g., in some implementations it is rightbehind the flexible membrane; in others, within about one or two inchesof the substrate). If the plate is fabricated from a conductivematerial, it may be a source of inaccuracy in the sensed eddy currentsignal. Therefore, fabricating such a plate from a non-conductivematerial may allow for more accurate determination of the eddy currentgenerated in the conductive regions on the substrate. Additionally, somecarrier head assemblies include conductive fasteners, even fornon-conductive parts of the carrier head. For example, retaining ringssimilar to retaining ring 1310 of FIG. 13 may be fabricated using anon-conductive material such as a hard plastic, for reasons unrelated toeddy current sensing. However, retaining rings are generally mounted tothe carrier head using conductive fasteners (not shown). Providing anon-conductive fastener may improve the accuracy of the eddy currentsensing system.

In other implementations of carrier heads, other elements may beproximate to a substrate being polished, and therefore (if conductive)may provide an unwanted contribution to a sensed signal for an eddycurrent monitoring system. Referring to FIG. 14, an implementation of acarrier head 1400 is shown. A description of a similar carrier head maybe found in U.S. Pat. No. 6,056,632, filed Oct. 9, 1998, issued May 2,2000, which is hereby incorporated by reference. Carrier head 1400includes a carrier plate 1410 proximate to a substrate being polished.Carrier head 1400 also includes fasteners such as fastener 1442, conduitfasteners 1431, and fasteners 1448 that are proximate to a substratebeing polished. Conductive plates and fasteners may affect the abilityof a eddy current sensing system to accurately reflect changes in aconductive layer on a wafer. Carrier plate 1410, which is both close tothe wafer and of a shape and size to produce significant eddy currentsunder the influence of a changing magnetic field, may be fabricatedusing a non-conductive material for improved eddy current sensing.Further, fasteners 1442, 1431, and 1448 may contribute to the eddycurrent signal if they are conductive. Signal improvement may beobtained by fabricating fasteners 1442, 1431, and 1448 using anon-conductive material.

Referring to FIG. 15, a carrier head 1500 includes a carrier body 1526proximate to a substrate being polished. A similar carrier head isdescribed in U.S. Pat. No. 6,443,820, which is hereby incorporated byreference. Like carrier plate 1410 of FIG. 14, carrier body 1526 is bothclose to the wafer and of a shape and size to produce significant eddycurrents under the influence of a changing magnetic field. Carrier head1500 also includes a number of fasteners such as fasteners 1510 whichmay affect eddy current sensing if they are conductive and/or magnetic.

FIG. 16 shows a plot of eddy current signal versus time as two differentversions of a chemical mechanical polishing system remove a thick copperlayer from a wafer. The first version includes a head with metal partswith a non-negligible magnetic permeability, while the second versionincludes a head with non-conductive, non-magnetic parts.Non-conductive/non-magnetic materials that may be used include plasticssuch as teflon and peek, aluminum (which may be electropolished and/oranodized), and non-magnetic steel. Many other materials may be used.

The difference in the sensed signal for the second version, denoted asΔ2, is much larger than the signal difference for the first version,denoted as Δ1. The metal parts, which are both conductive and magnetic,introduce a large background in the measured signal. Thus, thesensitivity of the eddy current sensing system in the first version issignificantly less than the sensitivity in the second version.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, differentimplementations of carrier heads and eddy current sensing systems may beused. Fabricating elements of the carrier head or other part of thechemical mechanical polishing apparatus from non-conductive materialssuch as plastics or ceramics may improve the accuracy of the eddycurrent sensing system. Accordingly, other embodiments are within thescope of the following claims.

1. A carrier head, comprising: a substrate backing assembly havingnon-conductive rigid components, the substrate backing assembly toprovide a mounting surface for a substrate to be polished.
 2. Thecarrier head of claim 1, further comprising a base assembly coupled withthe substrate backing assembly, wherein components of the base assemblywith at least a portion positioned within a sensing distance of themounting surface are nonconductive.
 3. The carrier head of claim 2,wherein the sensing distance is between about one tenth of an inch andabout two inches.
 4. The carrier head of claim 2, wherein the baseassembly includes a non-conductive gimbal mechanism to permit the baseassembly to move with respect to a housing.
 5. The carrier head of claim1, further including a non-conductive retaining ring secured to the baseassembly with one or more non-magnetic fasteners.
 6. The carrier head ofclaim 5, wherein the non-magnetic fasteners are also non-conductive. 7.The carrier head of claim 1, wherein the non-conductive rigid componentsare also non-magnetic.
 8. The carrier head of claim 1, wherein thesubstrate backing assembly includes a non-conductive plate proximate toan upper surface of a flexible membrane.
 9. The carrier head of claim 8,wherein the non-conductive plate is selected from the group consistingof a plastic plate and a ceramic plate.
 10. The carrier head of claim 1,wherein the substrate backing assembly includes a flexible membrane andone or more clamp rings.
 11. The carrier head of claim 1, furthercomprising a housing coupled with the substrate backing assembly via abase assembly, wherein components of the housing with at least a portionpositioned within a sensing distance of the mounting surface arenonconductive.
 12. The carrier head of claim 11, wherein the sensingdistance is about one tenth of an inch.
 13. The carrier head of claim11, wherein the sensing distance is between about one tenth of an inchand about two inches.
 14. A carrier head comprising: a housing to besecured to a drive shaft, the housing including one or more housingelements; a base assembly, the base assembly including one or more baseelements, the one or more base elements including means for permittingthe base assembly to move with respect to the housing; and a flexiblemembrane secured to the base assembly, the flexible membrane having alower surface that provides a substrate-mounting surface, and whereinthe base elements having at least a portion within a minimum distance ofthe lower surface are non-conductive, and further wherein the housingelements having at least a portion within the minimum distance arenon-conductive.
 15. The carrier head of claim 14, wherein the minimumdistance is about one tenth of an inch.
 16. The carrier head of claim14, wherein the minimum distance is between about one tenth of an inchand about two inches.